To counterfeit means to imitate something authentic, with the intent to steal, destroy, or replace the original, for use in illegal transactions, or otherwise to deceive individuals into believing that the fake is of equal or greater value than the real thing. Counterfeit products are fakes or unauthorized replicas of the real product. Counterfeit products are often produced with the intent to take advantage of the superior value of the imitated product. The word counterfeit frequently describes both the forgeries of currency and documents as well as the imitations of items such as clothing, handbags, shoes, pharmaceuticals, automobile parts, unapproved aircraft parts, watches, electronics and electronic parts, software, works of art, toys, and movies.

A replica is an exact copy or remake of an object, made out of the same raw materials, whether a molecule, a work of art, or a commercial product. The term is also used for copies that closely resemble the original, without claiming to be identical. Copies or reproductions of documents, books, manuscripts, maps or art prints are called facsimiles.